The present invention relates to chemical processing and solvent cleaning tanks, and more particularly to an apparatus and method for abating discharges of chemicals into the environment.
Chemical processing and solvent cleaning tanks have long been used to treat parts or materials. The process typically involves immersing the parts or materials to be treated, called the workload, into a processing tank containing a chemical or solvent bath. The processing tank typically has an open top to permit easy ingress and egress of the workload. The workload dwells in the processing tank for a period of time sufficient to complete the treatment. The workload is then removed from the processing tank and allowed to drain. From there, the workload may undergo additional treatments in separate processing tanks. The specific chemicals or solvents employed, and the workload dwell time required, are determined by the type of treatment process used and are well known within the art.
Because of the volatile nature of the chemicals and solvents typically employed, and the open design of most chemical processing tanks, vapors and mist emanating from the processing tank are discharged into the atmosphere. These chemical vapors often present hazards to both workers and the environment. In addition to health hazards, many solvents present fire and explosive hazards. Moreover, chemicals and solvents discharged into the atmosphere are difficult to reclaim and will consequently be counted as pollutants entering the waste stream. As a result, additional chemicals and solvents must be manufactured or purchased to replace those lost to the waste stream.
Chemical processing and solvent cleaning tanks are often heated above ambient temperature to increase the effectiveness of the treatment process and to reduce dwell time. Unfortunately, raising the temperature of the chemicals or solvents significantly increases their volatility, thereby increasing atmospheric emissions. The processing tank may also be agitated to increase the effectiveness of the treatment process. Agitation will likewise increase emissions.
The simplest but least effective prior art method for reducing vapor emissions consists of increasing the processing tank "freeboard." The freeboard is that portion of the tank sidewalls which extends above the chemical or solvent bath. The freeboard provides, within the confinement of the tank sidewalls, a zone in which vapors emanating from the bath can begin to cool. Hopefully, some of the vapors will recondense into a liquid within this zone and fall back into the processing tank.
A second prior art method for reducing vapor emissions involves installing cooling coils along the freeboard of the sidewalls just below the top of tank lip opening. The cooling coils aim to create a cold air blanket across the top of the tank opening. This cold air blanket increases the recondensation of the vapors emanating from the bath and further reduces emissions.
A third prior art method for reducing vapor emission involves adding a push-pull ventilation system just above the tank lip. The ventilation system attempts to capture any vapors which have not recondensed within the freeboard. The captured vapors are then directed to an exhaust system. In turn, the exhaust system may be equipped with pollution abatement equipment such as a scrubber or a carbon absorption device to reduce or prevent emissions into the atmosphere. Ventilation systems are typically installed over processing tanks where exposure to the chemical vapors would be harmful to the worker.
Finally, a lid can be used on the top of the processing tank to prevent emissions. But, the lid must be removed each time the workload is placed in, or removed from, the processing tank, thereby releasing chemical vapors into the atmosphere.
The prior art methods described above can be used separately or in any combination to reduce the amount of atmospheric emissions. However, none of the above methods, or combinations thereof, will provide for complete abatement of chemical vapor emissions. Nor do any of the above methods address the problem of fire or explosive hazards. In addition, there are a number of problems with the prior art methods which greatly reduce their effectiveness.
In all of the above prior art methods, air is utilized as the thermal transfer medium. It is well known within the art that air is a very poor thermal conductor. As a result, increasing the processing tank freeboard alone in order to contain all of the chemical vapor emissions would require raising the tank lip above the bath a distance equal to several times the length of the tank opening. A processing tank of such a configuration would be impractical to use.
For cooling coils to be effective, they must be capable of generating a continuous cold air blanket across the entire processing tank opening. To achieve this, the operating temperature of the cooling coils must be set extremely low (-10 degrees Fahrenheit is presently considered "Best Available Control Technology"). This leads to several problems. First, low operating temperatures will cause atmospheric water vapor to condense and contaminate the chemical or solvent bath. Where solvents are involved, water is an extremely undesirable contaminant. Second, low operating temperatures lead to the freezing of chemical and atmospheric water vapors, resulting in the formation of ice on the cooling coils. This leads to a loss of efficiency which causes the air blanket to warm. Eventually, the blanket will develop a "hole" through which chemical vapors may escape into the atmosphere.
Finally, ventilation systems do little to abate chemical vapor emissions. Their primary function is to protect workers from exposure to the chemical vapors.